Factors Controlling Water Quality

The quality of ground water primarily refers to the type and
concentration of dissolved substances in the water. These
substances can be dissolved gases and inorganic and organic
solids. In addition, particulates that might accompany water
flowing from a well may be of concern. These include sediment
from the subsurface formation, microorganisms, and chemical
precipitates that might form as a result of the disturbances
brought about by the well. The concentrations of the dissolved
and particulate substances are of interest because they can
affect the water use or operation of the well. One of the most
common considerations is whether the concentrations of
constituents fall below levels mandated or recommended for
drinking-water supply. Table 1 lists the maximum and recommended
concentrations for common major, minor, and trace constituents
naturally present in Dakota aquifer water. The concentrations
are values listed by the Kansas Department of Health and
Environment and were adopted mainly from the US Environmental
Protection Agency.

Natural rainfall contains dissolved air and a very small amount
of dissolved solids. During the travel of rainfall through soil,
sediment, and rock to reach an aquifer, the water dissolves
additional solids. Most of the soluble solids picked up by
the water are inorganic because inorganic minerals are the main
component of soils, sediments, and rocks, and natural organic
substances tend to be less soluble than common mineral salts.
Water flowing deeper into the subsurface may encounter very
soluble minerals such as rock salt and produce saltwater after
dissolving the salt. Other deep subsurface waters can be
highly saline because they were originally trapped seawater
that has been further altered through geologic time.

The total dissolved solids content (often abbreviated TDS) is
a general measure of the salinity of a water. The most common
substances contributing to the dissolved solids of most ground
waters are the inorganic constituents calcium, magnesium, sodium
(positively charged cations) and bicarbonate, chloride, and
sulfate (negatively charged anions). Bicarbonate may also be
represented as alkalinity in some water analyses. Inorganic
constituents commonly contributing minor amounts to ground-water
TDS are silica (uncharged), potassium (a cation), nitrate and
fluoride (anions). There are a large number of other individual
and combined elements dissolved in water, including gases and
metals. Table 2 lists the names, properties, and chemical symbols
or representation for common major and minor dissolved inorganic
substances as well as several dissolved metals in Dakota aquifer
waters. Concentrations of these dissolved substances are often
reported as milligrams per liter (abbreviated mg/L). This
concentration unit is essentially the same as a part per million
(ppm) in freshwater, because the weight of a liter of water
with dilute concentrations of dissolved constituents at at
ground-water temperatures is very close to 1,000 grams (one
million milligrams). In saltwater, a mg/L is a few percent
different from a ppm because the density of the solution is
greater than for freshwater. Concentrations of trace amounts
of dissolved constituents are often listed as micrograms per
liter (mg/L) which is very close to parts per billion (ppb)
in freshwater.

Freshwater is often defined as water containing less than
1,000 mg/L TDS. Freshwaters in the outcrop and subcrop portions
of the Dakota aquifer are usually calcium-bicarbonate or calcium,
magnesium-bicarbonate type waters. Most soils and near surface
rocks in Kansas, including the Dakota aquifer, contain at least
small amounts of calcium carbonate present as calcite (CaCO3).
The calcite contains small amounts of magnesium and the mineral
dolomite (CaMg(CO3)2) can also be present. During infiltration of
rainfall, the carbonate minerals dissolve and add calcium, magnesium,
and bicarbonate to the water. Small amounts of other inorganic
constituents are also dissolved from soils and near surface rocks.
These substances are present in the main carbonate minerals and
in the small amounts of other soluble minerals, are adsorbed on
clays, or have been concentrated as salts in soils during dry
periods. Typical ranges of major constituents and fluoride
concentrations in the most common chemical types of Dakota waters
are listed in Table 2.

Fine-grained sediments in the Dakota aquifer and overlying rocks
often contain the mineral pyrite (FeS2). The pyrite weathers to
produce dissolved iron and sulfate. The iron can then oxidize
and precipitate as iron oxide and oxyhydroxide (hydrated oxide)
which produces the red to orange coloration commonly occurring
in Dakota strata. The solution from pyrite weathering is acidic
and dissolves additional calcite in a natural neutralization
process. These processes increase the calcium and sulfate
concentrations dissolved in Dakota waters. Rocks overlying
the Dakota aquifer such as the Graneros Shale often include
gypsum (CaSO4 . 2H2O), a very soluble mineral. Water infiltrating
through these rocks can have relatively high concentrations of
calcium and sulfate from dissolving gypsum. Recharge passing
through rocks with gypsum and entering Dakota strata can
substantially increase the calcium and sulfate content of waters
in the upper aquifer. In some cases calcium-sulfate waters may
result, although this water type is not as common as other
chemical types in ground water from the Dakota (Table 3).

Large areas of the Dakota aquifer contain saltwater (primarily
dissolved sodium and chloride). Concentrations of TDS can be
in the ten's of thousands of mg/L (Table 2). Geochemical tests
have identified the main source of this saltwater as dissolution
of rock salt (NaCl) in Permian rocks underlying Dakota strata.
Although most of the Dakota sediments probably contained seawater
either during their deposition (the marine shales and sandstones)
or after deposition when the sea covered these units, nearly all
of the seawater has been flushed out by surface recharge. However,
saltwater from the underlying Permian rocks has been slowly
intruding into Dakota strata for millions of years. The
salt-dissolution brine replaced the seawater source of salinity
long ago. During more recent geologic time, freshwater recharge
has been slowly flushing saltwater from the Dakota aquifer.

The past occurrence of saline water in Dakota aquifer strata
resulted in the adsorption of large amounts of sodium on the
clays in the shales, siltstones, and sandstones. As freshwater
of calcium-bicarbonate type slowly flushed the saline water
from the aquifer, the process of natural softening of the water
occurred as dissolved calcium and magnesium adsorbed on the clays
and released sodium to solution. The decrease in calcium
concentration allowed some calcite to dissolve where present
in aquifer strata, thereby supplying additional calcium and
bicarbonate to the water. The added calcium was then available
for more cation exchange with sodium. Some additional bicarbonate
may have been generated from slow oxidation of organic matter
trapped in the aquifer framework. The combined effect of these
processes increased dissolved sodium and bicarbonate concentrations
while decreasing dissolved calcium, magnesium, and chloride
concentrations in confined parts of the Dakota aquifer where
the water is now fresh to slightly saline. The water types range
from sodium-bicarbonate to sodium-chloride, bicarbonate to
sodium-chloride with excess sodium in the direction of
increasing salinity. These waters are typically soft because
the calcium and magnesium concentrations are relatively low.
Typical ranges of major dissolved constituents in sodium-bicarbonate
waters in the Dakota aquifer are listed in Table 2.

The pH (a measure of how acidic or alkaline a water is) of the
sodium-bicarbonate waters in the confined Dakota aquifer is
alkaline and usually in the 7.5 to 8.5 range. (The pH of a water
indicates how acidic or alkaline is a water; a value of 7.0 units
represents a neutral solution at room temperature.) Elevated
concentrations of dissolved fluoride are also usually associated
with the sodium-bicarbonate waters. Dissolved fluoride contents
can be from over 1 mg/L up to several or more mg/L in comparison
with less than 1 mg/L for calcium-bicarbonate type waters. The
high fluoride derived from calcium minerals containing fluoride
(probably mainly apatites). The low calcium concentration resulting
from the cation exchange that produced the sodium-bicarbonate water
allowed the calcium minerals to dissolve. Some fluoride adsorbed
or weakly attached to clays was released in the higher pH waters by
exchange with hydroxyl ion (OH-).

Other naturally occurring constituents of interest in Dakota
aquifer waters are iron and manganese. Dissolved concentrations
of iron range from less than a few mg/L to over 10 mg/L and
manganese range from less than a mg/L to nearly a mg/L. The
greater concentrations occur in two types of environments. One
occurrence is in the outcrop or subcrop area of the Dakota aquifer
where recharge with dissolved oxygen reaches strata containing
pyrite. Oxidation of pyrite was referred to earlier as a source
of sulfate as well as dissolved iron in ground waters. Such waters
can have a pH between 6 and 7 (very slightly acidic). The other
occurrence exists where reactions with dissolved constituents and
sediments have essentially completely consumed dissolved oxygen and
produced a chemically reducing environment. This commonly occurs
in the confined portion of the Dakota aquifer because the age of
the water is old and no recent recharge with significant oxygen
can enter. The reducing environment allows iron, manganese, and
some other heavy metals to dissolve from the sediments. These
waters can sometimes have a high enough hydrogen sulfide (H2S)
content to give a "rotten egg" odor. Ammonium ion (NH4+) levels
can be over a mg/L in the reducing environment (Table 1).